Some refrigeration applications, including transport refrigeration, require operation at reduced capacity to hold product within a very narrow temperature range. In some cases suction modulation is used to reduce and regulate capacity. This affects suction and discharge temperatures. When suction modulation occurs at high ambient temperatures, the refrigerant supplied to the compressor may be too hot, absent some correcting measures, and this results in compressor discharge temperatures that are too high. If discharge temperatures are not kept from getting too hot, the compressor lubricant can break down and ultimately cause failure of the compressor.
Liquid refrigerant is often used to lower the discharge temperature by feeding it into the suction side of the compressor. One approach is to operate a solenoid valve responsive to the suction modulation valve. This approach is not responsive to ambient or any other temperature reference and can provide unwanted quench as at low ambient and low discharge temperature. Too much liquid refrigerant can also result in liquid slugging or floodback to the compressor and can ultimately cause failure of the compressor.